Steel used for a liquefied natural gas (LNG) tank needs to have fracture-resisting performance at an extremely low temperature of approximately −160° C. For example, 9% Ni steel is used for the inside tank of the LNG tank. The 9% Ni steel is a steel material that contains, by mass %, approximately 8.5% to 9.5% of Ni, has a microstructure mainly including tempered martensite, and is excellent in, particularly, low-temperature toughness (for example, Charpy impact-absorbing energy at −196° C.). Various techniques to improve the toughness of the 9% Ni steel have been disclosed. For example, Patent Documents 1 to 3 disclose techniques in which P that causes a decrease in toughness due to intergranular embrittlement is reduced. In addition, Patent Documents 4 to 6 disclose techniques in which tempering embrittlement sensitivity is reduced using a two-phase region thermal treatment so as to improve the toughness. Additionally, Patent Documents 7 to 9 disclose techniques in which Mo that can increase strength without increasing the tempering embrittlement sensitivity is added so as to significantly improve the toughness. Furthermore, Patent Documents 4, 8, and 10 disclose techniques in which the amount of Si that increases the tempering embrittlement sensitivity is reduced so as to improve the toughness. Meanwhile, a steel plate having a plate thickness of 4.5 mm to 80 mm is used as the 9% Ni steel for the LNG tanks. Among them, a steel plate having a plate thickness of 6 mm to 50 mm is mainly used.
Due to a current increase in the price of Ni, there is a demand for a steel material in which the addition of Ni is reduced in order to reduce the manufacturing costs of the LNG tanks. As a method in which the addition of Ni in the steel material is reduced to 6% so as to secure excellent base metal toughness, NonPatent Document 1 discloses a method in which a thermal treatment in an α-γ two-phase region (two-phase region thermal treatment) is used. The method is extremely effective in improving the fracture-resisting performance of base metal. That is, in spite of an amount of Ni being approximately 6%, a steel material obtained using the method has the same fracture-resisting performance (toughness described below) as the 9% Ni steel in terms of the base metal. However, in accordance with reduction of the amount of Ni, the fracture-resisting performance (toughness, arrestability, and unstable fracture-suppressing characteristic described below) of a welded joint significantly degrade. Therefore, it is difficult to use the steel material manufactured using the above method for the LNG tanks.
Hitherto, several methods to improve the fracture-resisting performance (toughness described below) of the welded joint have been proposed. For example, Patent Documents 11 to 14 disclose methods in which a preliminary thermal treatment for reducing segregation is carried out before a cast slab is heated and rolled. In addition, Patent Document 15 discloses a method in which two processes of rolling are carried out so as to decrease defects in a plate thickness central portion. However, in the method of Patent Documents 11 to 14, since the effect of segregation reduction is small, the fracture-resisting performance (toughness described below) of the welded joint is not sufficient. In addition, in the method of Patent Document 15, the rolling reduction ratio of the plate thickness after the final rolling to the plate thickness of the cast slab is small, and conditions such as the rolling reduction or temperature in the first rolling process are not controlled. Therefore, the fracture-resisting performance (toughness described below) of the base metal and the welded joint is not sufficient due to microstructure coarsening and segregation remaining. As such, it is difficult to secure the fracture-resisting performance at approximately −160° C. in the steel plate in which the amount of Ni is reduced to approximately 6% using the existing techniques.